1. Field of the Invention
The present invention relates to matrix substrates used in liquid crystal devices, and relates to liquid crystal display devices and projection liquid crystal display devices using the matrix substrates.
2. Description of the Related Art
Recent progress in information networks has increased the need for display devices for communication of information, particularly image information. Liquid crystal display devices, which are thin and have an advantage in low electrical power consumption, have attracted considerable attention and are growing as one of the basic industries, similarly to the semiconductor industries. Recently, liquid crystal display devices have mainly been used in 12xe2x80x3 notebook personal computers. In the future, liquid crystal display devices having larger screen sizes will be used in workstations and home televisions, as well as in personal computers. The trends toward increasing scale in liquid crystal display devices, however, demands the introduction of expensive apparatuses for producing such devices. Further, large scale liquid crystal display devices must have extreme electrical characteristics for driving large screens. Thus, production costs increase significantly, that is, in proportion to from the square to the cube of the screen size.
A front- or back-projection system using a small-size liquid crystal display panel recently has attracted attention in which a liquid crystal image is optically enlarged and displayed. Performance and production costs of liquid crystal display devices are improved with size reduction in the devices by the scaling rule as in semiconductors. In TFT liquid crystal display panels, TFTs using polycrystalline Si are being substituted for those using amorphous Si to meet the requirement of small-size TFTs having high driving force. Image signals having a resolution level in the NTSC standard do not require high-speed processing.
A possible TFT liquid crystal display device which can be used to meet such requirements has an integrated structure including a display region and peripheral driving circuits, such as a shift register and a decoder, which are also formed of polycrystalline Si. Polycrystalline Si, however, is not comparable to single crystal Si. When a display of an extended graphics array (XGA) or super extended graphics array (SXGA) class in the resolution standard of computers is designed, for example, the shift register must inevitably be divided into a plurality of segments. Signal noise (ghosting) will occur in the display region corresponding to the boundary between the segments. Countermeasures are required for solving such problems.
Japanese Patent Laid-Open No. 9-73103 discloses a reflection-type liquid crystal device using polycrystalline Si and single crystal Si for solving the degradation problem of the displayed image by a decreased contrast which is caused by reduced reflectance of light because of scattering of light incident on the pixel electrode having surface unevenness in all directions, and insufficient alignment of the alignment film in the rubbing step in the liquid crystal mounting process and thus unsatisfactory alignment of the liquid crystal.
Japanese Patent Laid-Open No. 9-73103 discloses pixel electrodes having smooth surfaces formed by chemical/mechanical polishing (hereinafter referred to as CMP). Each of the resulting pixel electrodes has a mirror surface and all the pixel electrodes lie in the same plane. The smooth surface permits display of high quality images free of random scattering of incident light and insufficient alignment of liquid crystals.
A method for making an active matrix substrate disclosed in Japanese Patent Laid-Open No. 9-73103 will be described with reference to FIGS. 30A to 30H. FIGS. 30A to 30H describe a pixel section. Peripheral circuits such as a shift register for driving a switching transistor in the pixel section can also be simultaneously formed during the formation of the pixel section on the same substrate.
An n-type silicon semiconductor substrate 201 having an impurity concentration of 10 15 cmxe2x88x923 or less is subjected to local thermal oxidation to form a LOCOS (local oxidation of silicon) layer 202, and boron ions are implanted in a dose of approximately 1012 cmxe2x88x922 through the LOCOS layer 202 as a mask to form a p-type well (PWL) 203 being a p-type impurity region having an impurity concentration of 1016 cmxe2x88x923. The substrate 201 is thermally oxidized to form a gate oxide film 204 having a thickness of 1,000 angstroms or less (FIG. 30A).
An n-type polysilicon gate electrode 205 is formed by doping phosphorus in an amount of approximately 1020 cmxe2x88x923, phosphorus ions are implanted onto the entire surface of the substrate 201 in a dose of approximately 1012 cmxe2x88x922 to form an n-type lightly doped drain (NLD) 206 being an n-type impurity region having an impurity concentration of 1016 cmxe2x88x923. Phosphorus ions are implanted through a patterned photoresist mask in a dose of approximately 1015 cmxe2x88x922 to form source and drain regions 207 and 207xe2x80x2 having an impurity concentration of approximately 1019 cmxe2x88x923 (FIG. 30B).
A phospho-silicate glass (PSG) 208, which is a phosphorus-doped oxide film, is formed as an interlayer on the entire substrate 201. The PSG film 208 can be replaced with a nondoped silicate glass (NSG)/boro-phospho-silicate glass (BPSG) film or a tetraethoxysilane (TEOS) film. Contact holes are patterned into the PSG film 208 just above the source and drain regions 207 and 207xe2x80x2. Aluminum is deposited by a sputtering process and then patterned to form an aluminum electrode 209 (FIG. 30C). It is preferred that a barrier metal composed of Ti or TiN be formed between the aluminum electrode 209 and the source and drain regions 207 and 207xe2x80x2 so as to improve the ohmic contact characteristics between the aluminum electrode 209 and the source and drain regions 207 and 207xe2x80x2.
A plasma SiN film 210 with a thickness of approximately 3,000 angstroms, and then a PSG film 211 with a thickness of approximately 10,000 angstroms are formed on the entire substrate 201 (FIG. 30D). The PSG film 211 is patterned using the plasma SiN film 210 as a dry etching stopper layer so as to leave the separation region between pixels, and then a through hole 212 is patterned just above the aluminum electrode 208 which is in contact with the drain region 207xe2x80x2 by dry etching (FIG. 30E).
A pixel electrode 213 with a thickness of approximately 10,000 angstroms or more is formed on the substrate 201 by sputtering or electron beam (EB) deposition (FIG. 30F). The pixel electrode 213 is composed of a metal film of Al, Ti, Ta or W, or a metal compound film of such a metal. The surface of the pixel electrode 213 is polished by CMP (FIG. 30G).
An alignment film 215 is formed on the resulting active matrix substrate, and its surface is subjected to alignment treatment such as rubbing. The substrate is bonded with a counter substrate with a spacer (not shown in the drawing) therebetween, and a liquid crystal 214 is injected into the gap to form a liquid crystal device (FIG. 30H). The counter electrode includes a transparent substrate 220, a color filter 221, a black matrix 222, a common electrode of ITO 223, and an alignment film 215, in that order.
The reflection-type liquid crystal device is driven as follows. Peripheral circuits including a shift register which is formed on the substrate 201 by an on-chip process applies a signal potential to the source region 207 and a gate potential to the gate electrode 205 such that the switching transistor in the pixel in an ON state supplies signal charge to the drain region 207xe2x80x2. The signal charge is accumulated in a pn-junction cavity capacitor formed between the drain region 207xe2x80x2 and the PWL 203 to impart a potential to the pixel electrode 213 through the aluminum electrode 209. The potential application to the gate electrode 205 is suspended when the potential of the pixel electrode 213 reaches a given value so that the pixel switching transistor is in an OFF state. The signal charge accumulated in the pn-junction capacitor fixes the potential of the pixel electrode 213 before the pixel switching transistor is redriven. The fixed potential of the pixel electrode 213 drives the liquid crystal 214 encapsulated between the substrate 201 and the counter substrate 220 shown in FIG. 30H.
The pixel electrode 213 of the active matrix substrate has a smooth surface as shown in FIG. 30H, and an insulating layer is embedded into the gap between two adjacent pixel electrodes. Thus, the alignment film 215 formed thereon has a smooth surface, which prevents a decrease in light efficiency due to light scattering, a decrease in contrast due to insufficient rubbing, and the formation of an emission line due to a horizontal electric field formed by a step between two pixel electrodes. As a result, the quality of the displayed image is improved.
An active matrix liquid crystal display device generally has a pixel electrode in the display section and a pixel switch for applying a desired potential to the pixel electrode, as in the above-mentioned configuration. A holding capacitor is generally provided to maintain the potential of the pixel electrode. In a typical embodiment, a conductive (lead) layer as a common potential lead is provided under the pixel electrode for stabilizing the potential of the pixel by capacitor coupling with the pixel electrode. The common potential lead can also be used as a shading layer of the pixel switch, and can also function as a shading layer of the transistor in the peripheral driving circuit. The potential of the shading layer is fixed to a predetermined value as described in an embodiment in Japanese Patent Laid-Open No. 9-73103.
In a typical configuration of a liquid crystal display device, the substrate with pixels and the counter substrate are bonded to each other with an adhesive or a sealant applied at the peripheral edges of the display section and a liquid crystal is displaced in the gap formed between the two substrates. Since the display characteristics and their uniformity depend on the gap between the two substrates, a gap having a desired distance must be formed with high accuracy. Such a gap distance can be controlled by the size of spherical particles (termed xe2x80x9cspacersxe2x80x9d) to be interposed between the substrates. The spacers may be fixed with the sealant and applied onto the substrate.
When the shading layer is arranged in the sealing section in which the sealant is applied, pressurizing of the spacers may cause mechanical rupture of the spacers in some cases, resulting in short-circuiting of the shading layer to other sections such as the substrate and the lead.
Accordingly, it is an object of the present invention to solve the above-mentioned problems by providing a matrix substrate.
It is an object of the present invention to provide a matrix substrate including a plurality of pixel electrodes arranged in a matrix, and a driving circuit region and a sealing region provided in the peripheral region of the plurality of pixel electrodes, a sealant, a spacer material and a liquid crystal material being disposed between the plurality of pixel electrodes and a counter substrate to constitute a liquid crystal display device. A first conductive layer is provided under the plurality of pixel electrodes, a second conductive layer is provided under the sealing region for arranging the sealant and the spacer material, and the first conductive layer is electrically separated from the second conductive layer.
It is another object of the present invention to provide a liquid crystal display device including: a matrix substrate comprising a plurality of pixel electrodes arranged in a matrix, and a driving circuit region and a sealing region provided in the peripheral region of the plurality of pixel electrodes; a sealant, a spacer material and a liquid crystal material being disposed between the plurality of pixel electrodes and a counter substrate. The matrix substrate is provided with a first conductive layer under the plurality of pixel electrodes, a second conductive layer is provided under the sealing region for arranging the sealant and the spacer material, and the first conductive layer is electrically separated from the second conductive layer.
Electrical separation between the first conductive layer and the second conductive layer can prevent a decreased yield. Such an advantage is further prominent when the second conductive layer under the sealing region is in a floating state.
Preferably, in the matrix substrate and liquid crystal display device in accordance with the present invention, a desired potential is applied to the first and second conductive layers.
Preferably, the matrix substrate is formed of a first conductive-type semiconductor substrate and a second conductive-type region is formed under the sealing region.
The second conductive-type region may be a floating region or may have the same potential as that of the first conductive layer. Alternatively, almost of the conductive layer under the sealing region may be floating.
A metallic layer of the same layer as the pixel electrode may cover the gap between the first conductive layer and the second conductive layer or between the second conductive layer and a conductive layer provided under the driving circuit region.
A glass sheet is preferably arranged on the counter substrate. A microlens group is preferably arranged on the counter substrate. An element of the microlens group is arranged to three of the plurality of pixel electrodes.
A projection-type liquid crystal display preferably uses the above-mentioned liquid crystal display device. Preferably, an image formed by said liquid crystal display device is projected by separating a blue light component using a high-reflectance mirror and a blue-light reflecting dichroic mirror and by separating a red light component from a green light component using a red-light reflecting dichroic mirror and a green/blue-light reflecting dichroic mirror.
The sealant and the spacer on the periphery of the liquid crystal display device hold the distance between the pixel electrode and the counter substrate constant. The conductive layers constituting shading layers, which are electrically separated from each other, will short-circuit to the substrate or a lead even when mechanical rupture occurs by the pressure of the spacer. The floating potential of the shading layer at the sealing section secures the stabilized operation of the device regardless of the pressure by the spacer and thus improves the production yield.
The projection-type liquid crystal display device in accordance with the present invention uses a reflection-type liquid crystal panel with a microlens, and an optical system for illuminating three primary light beams from different directions, so that modulated reflecting light beams from a suit of RGB pixels for one pixel unit are emitted through the same microlens element. Thus, the display device can display a high-quality image free of an RGB mosaic pattern.
The light beams from each pixel are substantially collimated by passing twice through the microlens. Thus, A bright projected image is displayed on a screen even when an inexpensive projection lens with a low aperture is used.
An image with having higher quality, higher brightness and a higher density is achieved by planarization of the pixel electrode enabling exact reflection of light beams.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.